My horse is starting to get a little "ribby"...thinking of using soybean oil?

You can't feed a horse whole soybeans as it is toxic to them. Soybean oil is what most vegetable oils are, so it is fine to feed it, and is pure fat and safer than feeding soluble starches which are associated with multiple metabolic conditions and degenerative joint disease. A cup of oil will provide 2000 calories. An average 1000 lb horse can be fed up to two cups of oil daily which will add 4000 safe calories. Start with no more than a quarter of a cup and very gradually increase the amount over a period of weeks. The body needs time to adapt to metabolizing the increased fat. If your horse develops diarrhea, cut back on the amount. This will fatten a horse up relatively rapidly, so take it slow. You can also cut back on cereal grains which are high in soluble starch and not as healthy as an energy source for your horse as the fats. Be sure your horse is getting plenty of long stem fiber from hay (mixed grass hay is lowest in soluble starches) as well.